JP2004518108A5 - - Google Patents

Description

A wireless DGPS receiver can report the position of the barge to an interrogator within range. The interrogator can be a tow ship, another manned ship, or the VTC itself. The tow ship maintains accurate recognition of its barge location and can relay its data to the VTC. A portable self-powered wireless DGPS receiver can be mounted on each barge to be tracked. The receiver must function in a neglected state, at least during the voyage, preferably over a longer period, and even infinitely. The environment of the barge to be tracked is less oscillating and moving, but it is expected that the receiver unit will fall or be tampered with while moving. The RF environment includes VHF marine band activity, urban RF noise, and marine radar transmissions. GPS signals with an elevation of 30 degrees or less are expected to be disturbed, but satellites at higher elevations are visible while the unit is operating in the barge to be tracked ( must be visible). In that environment, there will be multiple routes from its main boat, other ships and vessels and coastal facilities.

At least three solutions are currently available to provide DGPS tags for objects that are not remotely powered, such as a barge. All of which would require a complete DGPS transponder, GPS pseudorange transponder (GPS pseudorange transponder) or battery system connected to existing technology such as RF sampling transponder.

GPS pseudorange transponder consists of a GPS receiver integrated with RF modem. In that transponder, navigation mark corrections are not collected. The receiver must be kept on in order to have a useful pseudorange on demand. When the unit is interrogated, it returns a pseudo distance towing ship. DGPS corrections are collected, stored on the towed vessel, and applied to the received pseudoranges. The barge position calculation is performed on a towed vessel. This system is also required that the device is continuously powered in order to make measurements on demand.

The RF sampling transponder frequency-converts and digitizes the L1 band signal, and further transmits the RF sample itself . There is no need to keep it on between calls . The processing of the RF samples for the code correlation operation is completely performed in the interrogator. Although not required only minimal power in this application, it is itself in terms of complexity and reliability has to transfer large amounts of data lead to problems.

According to a detailed embodiment of the present invention, the step of generating a correlation snapshot obtains a non-coherent sum of a plurality of integrals using a plurality of correlators positioned away from a certain chip. A step of determining an approximate signal peak value from a non-interference sum, pre-positioning a correlator at a code phase predicted from the signal peak, and an eighth chip to generate a sum of a plurality of correlators Performing integration on the intervals of the correlators. According to a more detailed embodiment, the non-interfering sum is obtained by two 1 millisecond integrations using 12 correlators spaced one chip apart, and the sum of multiple correlators is generated by 10 millisecond integration. Is done.

Returning to FIG. 1, the GPS tag system 10 includes an interrogator 12 disposed on a base object 22 and at least one transponder 14 (hereinafter referred to as a remote object 16 disposed on a location where power is available or unavailable). , Also referred to as “tag”). Interrogator 12 includes a receive antenna 18 for receiving GPS signals 20 transmitted by one or more GPS sources, such as satellites. The interrogator 22 further includes a wireless antenna 24 for communicating with the transponder 14. The transponder 14 also includes a receiving antenna 28 for receiving the GPS signal 20 and a wireless antenna 30 for communicating with the interrogator 12. The interrogator 12 and the transponder 14 transmit and receive the RF signal 26 in both directions . The contents of these RF signals 26 will be described later.

All signals from the interrogator 12 to the transponder 14 are started with an activation signal. This is an unmodulated carrier that is transmitted for about 10 milliseconds, possibly at a higher power than the rest of the signal. The purpose of the activation signal is to wake up all of the transponders 14 that are in the signal transmission range. This signal wakes up the transponder 14 from a passive standby state. The rest of the signal is the synchronization pattern and ID for the transponder 14 to be polled. According to the simplest embodiment, signals between three interrogators-transponders are defined. The first signal is widely transmitted to all transponders 14 to identify themselves. The second signal is a response signal for terminating the identification signal of the transponder 14 (identification message) (acknowledge message ). Finally, an interrogation signal 32 is also referred to as a “correlation snapshot command signal”. In the case of a correlation snapshot command signal, the synchronization pattern and ID are followed by pre-positioning data and a tracking signal.

The entire process of obtaining a correlation snapshot and returning data to the interrogator 12 is performed in the range of 40 to 50 milliseconds. For a given frequency of inquiry, the transponder 14 is powered for only 50 milliseconds per second. This low duty cycle contributes to a reduction in the power used by the GPS tag system 10, in other words, greatly increases battery life and, in particular, contributes to known systems that require continuous power. The transponder correlation snapshot command signal 32 provides all of the information required for the transponder 14 to find the GPS signal and constitutes a correlation snapshot 34 for returning to the interrogator 12. The data of the correlation snapshot command signal 32 includes a PRN code number, a chip number, and a Doppler offset to be received by each GPS signal source. The correlation snapshot command signal 32 is followed by a tracking signal to give the transponder 14 a reference time and frequency characteristic. The frequency characteristics are brought about by the incoming carrier. The timing information is based on a code given to the tracking signal. A short continuous code after the pre-position information indicates a GPS data epoch. The correlator channel of each transponder 14 must begin acquiring signals on the commanded code chip during a fixed time after the data epoch occurs, as shown in FIG.

The GPS system 10 optimizes the division of processing between the tag 14 on the remote object such as the boat 16 and the interrogator 12 on the base object such as the tow ship 22, thereby reducing the power consumption of the transponder. Minimize. The present invention, any information may not be displayed in the transponder 14, or not used, therefore, the process of the transponders in the correlation operation for replicating code collection and its GPS signal RF samples of GPS signals 20 (code replica) limited to.

As described above, the interrogator 12 transmits an RF signal including the challenge signal 32 to the transponder 14. In response, each of the transponders 14 on the boat 16 transmits an RF signal including a correlation snapshot 34 to the interrogator 12. Using information including the correlation snapshot 34, the interrogator 12 performs the processing necessary to determine the properties and pseudorange voyage transponder 14 (pseudorange), whereby the transponder is placed Provides the position and velocity of the object 16.

With respect to the boat towing system, the transponder 14 is placed on each boat 16 in the convoy before starting the voyage. The transponder 14 is preferably mounted ready for use on the highest level flat metal surface. Once placed on the boat 16, the transponder 14 is manually switched from off to on. When switched on, the transponder 14 enters a standby state. As described below, the transponder 14 includes a passive standby circuit and maintains the transponder in a standby state until an interrogation communication 32 is received.

After placement of the transponder 14, the operator switches on the interrogator 12. Once activated, the interrogator 12 starts capturing the GPS signal 20. The GPS tag system 10 further includes a user interface 36 that communicates with the interrogator 12. Through the user interface 36, the operator can select an identification tag (IDENTIFY TAGS) mode. When this mode is selected, the interrogator 12 transmits a BPSK modulated identification request signal (not shown) received by all transponders 14. This request signal is separated and distinguished from the challenge signal 32. Each transponder 14 transmits a response signal (not shown) after an internally generated random delay. Response signal of the transponder 14 is the same frequency, including 20-bit ID code that has been BPSK modulated on the signal. The interrogator 12 recognizes reception of the response signal of the individual transponder 14. Each transponder 14 continues to repeat any time delay and ID transmission until the ID is recognized or a timeout is reached. Although a transponder response signal collision may occur, the identification operation is still being performed as each transponder 14 continues to transmit until recognized by the interrogator 12. Once a transponder response signal is received, the user interface 36 then displays a list of all transponders 14 in the population. The operator can give a custom name to any of the tags 14, which are characterized by a 5-digit hexadecimal ID code by default. No two transponders 14 have the same ID code.

According to one embodiment of the present invention, the operator selects a tracking mode at interface 36 and selects a polling rate for updating the position of each transponder 14. The interrogator 12 initiates a round robin polling of the transponder 14 and obtains a set of six satellite correlation snapshots 34 from each transponder. The interrogator 12 calculates the positions of the tags 14 and displays them on the user interface 36. The GPS tag system 10 also provides a warning radius that will sound a warning when any of the transponders 14 gets a signal from the interrogator 12 that is closer or farther than the selected distance. The interrogator 12 periodically transmits an identification request signal to recognize whether another transponder 14 has entered the range. When the first towing vessel navigates to the route occupied by the second towing vessel, the transponder 14 from the boat towed by the second towing vessel is transferred to the interrogator 12 mounted on the second towing vessel. Can respond. The interrogator 12 mounted on the first towed vessel receives these signals and plots the position of the transponder. Detection of new transponders 14, minimum or maximum distance violations between the boat and the boat, the maximum distance exceeds the limit of the boat and the towing vessel, and, by warning definitive in situations such as loss of ship responses operator Can be set.

At the destination, the boat 16 is moored and the interrogator 12 is set to the standby state. The GPS receiver of the interrogator 12 remains on to accommodate the early start of the next voyage. The interrogation of the transponder 14 stops. The transponder 14 automatically returns to the standby state when not called. As already mentioned, the transponder consumes less than milliwatts of power when it enters standby .

With reference to FIG. 2, the transponder 14 includes a power subsystem that includes an antenna switch 42, a passive standby circuit 46, a power supply control 48, and a solar power storage controller 50 . Antenna switch 42 is shown in a normally closed state, the passive standby circuit 46 is powered by the receiver 30. During the sleep state, the transponder 14 is basically powered down by the receiver 30 supplying power to the passive standby circuit 46. Passive standby circuit 46 is a synchronized filter that includes a precision diode detector, a low pass filter, and a comparator that drives the gate signal of MOSFET power supply controller 48. With the RF signal at the resonant frequency, the built-in low-pass filter generates a DC output voltage until the low-pass filter passes a threshold and trigger to switch the power supply on to the comparator. The only current consumption in the standby state is the bias current for the precision diode detector and the current consumed by the comparator in the off state. In most embodiments, this should total only less than 1 milliwatt and can be reduced to tens of microwatts by designing an appropriate diode detector. The operation of this circuit varies depending on the frequency band selected for GPS tag system communication. The low sensitivity of the detection circuit can be compensated by increasing the power of the interrogation signal 32 only for the starting portion of the waveform.

According to another embodiment, a relatively advantageous passive standby circuit that relies on the presence of two tones above the background noise before the trigger is used to reduce the possibility of false triggering of the transponder 14 Can be done. The advantage of this approach is that it reduces the false alarm rate , thereby saving power. In this case, the circuit is implemented by tripling passive tuned filter receivers. Two of these are used to detect continuous wave (CW) tone signals from the interrogator 12. Among these, the third receiver is used for measurement of noise and disturbance in the focused frequency band. The three output voltages are captured in a pair of comparators that control the gate voltage of the power MOSFET. The third receiver channel performs the same function as the automatic gain control (AGC) in the active receiver and increases the dynamic range of the standby receiver.

The correlator 54 receives the carrier frequency from the PLL / VCO 40 and receives preposition information from the microcontroller 60. The correlator 54 performs a correlation operation on the GPS signal received by the antenna 28 and processed by the GPS RF / IF 58 and the ADC 56. The sum of the correlation operations is provided to the microcontroller 60 and forwarded to the RF communicator 52 for transmission to the interrogator 12 as a correlation snapshot.

As described above, the transponder 14 (see FIG. 1) is designed to return a correlation snapshot 34 to the interrogator 12. Correlation snapshot 34 is a sampling between the received GPS signal 20 and a replica generated at tag 14 at a constant offset of the fraction of the chip, at least for the entire chip range. The obtained path acquisition (C / A) code correlation function. Correlation snapshot 34 is obtained as a set of correlator outputs summed over an integration interval and transmitted as a set of fixed point values. Correlation snapshot 34 is formed in response to interrogation signal 32 to account for code phase and frequency offsets. A typical challenge signal 32 consists of the requested pseudo-random noise (PRN) code number, the code phase offset within the chip, and the Doppler frequency offset of the GPS signal 20. The transponder 14 responds with a distance offset (a range offset) in the set and the chip correlators sum of fixed-point. In one embodiment, correlation snapshot 34, each output shows the 1 offset-eighth of a chip, includes a 12 correlator outputs of four bits. Since the transponder does not remain on long enough to track the GPS carrier, the GPS carrier is not locked at the transponder 14. Instead, accurate GPS signal carrier frequency information is provided by the RF carrier from interrogator 12, which allows coherent integration without a GPS carrier tracking loop. The bandwidth and correlator spacing of the transponder 14 are sized so that the code phase noise can be guaranteed to be less than 1 meter at one sigma.

According to one embodiment, the correlator 54 of the transponder 14 includes 72 correlators grouped into six channels. Each channel uses 12 correlators to obtain a correlation snapshot 34 of a single GPS signal 20. Correlation snapshot 34 is generated in three steps. Referring to FIG. 3, the first step S1 comprises using twelve correlators located away at one chip intervals, asynchronous sum of two one millisecond integrated (noncoherent sum). Asynchronous sums are used to maintain a wider bandwidth than in the case of a single synchronized 2 millisecond integration. This prepositioning error of the code phase is less than ± 1758 meters and, when prepositioning error of the carrier frequency is less than ± 375 Hz, to determine the location of the GPS signals 20.

In the second step, all 12 correlators are pre-positioned with the code phase predicted from the signal peak in step S1. Each correlator is separated from the Doppler frequency at the expected 62.5 Hz interval. A 2 millisecond integration is performed. The output of these correlators is compared at the end of step S2, the Doppler frequency is predicted in the range of 32.25Hz as a worst case of Step S3. In step S3, the 12 correlators perform 10 millisecond integration at 1/8 chip correlator intervals. The correlator sum is normalized by the transponder microcontroller and returned to the interrogator as a set of twelve 4-bit values along with the chip offset calculated in step S1.

Interrogator 12, for the evaluation of the calculation and a plurality of paths of pseudorange (pseudorange), to analyze the correlation snapshot 34. Narrow correlator spacing, by evaluating the primary and delayed ray (delayed rays) at the interrogator 12, substantially to remove some of the plurality of paths in a short distance. It should be noted that the entire correlation snapshot 34 is generated within the interval of one GPS data bit, i.e. within 20 milliseconds. The interrogator 12 sends a signal to the transponder 14 to initiate a correlation within a few milliseconds of the data bit boundary. Since the data bits are not decoded by the transponder 14, integration across the data bit boundaries is avoided and removal from the potential correlator sum is prevented.

According to an alternative embodiment, the transponder 14 comprises only one channel with 12 correlators. In the present embodiment, the transponder 14 is continuously polled in order to obtain correlation information of each GPS signal 20 viewed from the interrogator 12. This slightly increases the processing required in the interrogator 12, but has the advantage that the transponder 14 can be simplified.

According to another alternative embodiment, the transponder 14 is acquired over a 10 millisecond sample by fitting a triangular correlation function to the data points representing the correlation curve from which the peak correlation position is calculated. Twelve correlator values are reduced to chip offset. Next, the correlator value is converted into fixed-point numbers are returned along with the initial whole chip offset calculated during step 1. Step 1 is an asynchronous sum of two 1-millisecond integrations using 12 correlators spaced one chip apart, where a full chip offset is established. This embodiment has an advantage that even less data can be returned to the interrogator 12. That is, 10 bits for 1 meter resolution of the peak correlation position, 4 bits for all chip offset, 14 bits in total for each satellite, and 84 bits for the 6-channel configuration.

According to the preferred embodiment of the GPS tag system 10, the transponder 14 returns twelve 4-bit correlator outputs for each satellite to the interrogator 12. For 6 satellites, 288 bits are transmitted. This is sufficient for one meter pseudorange accuracy and allows the GPS tag system 10 uses multiple routes mitigation techniques.

To perform the measurement of the signal detection and pseudorange in a single GPS data bit, the transponder 14 must have precise frequency information. FIG. 4 shows the carrier frequency error versus correlation loss for 1, 2, and 10 millisecond integration intervals. The Doppler preposition data in the interrogation signal 32 does not cause a frequency error in the transponder 14 clock and is related to a nominal L1 frequency of 1575.42 MHz. The transponder 14 tracks the carrier frequency of the interrogation signal 32 using the PLL / VCO 40 to set the frequency of the local code generator. This allows accurate frequency information to be transmitted from an interrogator oscillator (not shown) included in the interrogator 12. Then, thereby, the transponder 14, without local GPS carrier tracking loop (local GPS carrier tracking loops) or expensive temperature compensated crystal oscillator (temperature compensated crystal oscillator (TCXO) ), the measurement of 10 ms correlation function Can be done .

As shown in FIG. 4, the initial frequency error for the 1 ms integration of the correlation snapshot 34 should be less than ± 375 Hz at L1, ie, less than ± 0.238 ppm, in order to maintain a correlation loss of −6 dB or more. I must. This can be achieved with a phase-locked loop that tracks the incoming interrogation signal 32. In the second step of the correlation snapshot, With 62.5Hz Doppler interval correlators, the system can determine the signal Doppler (Signal Doppler) in the range of 31.25 Hz. Referring again to FIG. 4, whereby, with the 10 ms integration, to maintain the correlation loss above -2 dB.

When the interrogator 12 generates an interrogation signal 32 for the transponder 14, it predicts the code phase that will be visible to the transponder. Transponder 14 samples the correlation function over 12 code chips in the summed 1 millisecond integration interval. Twelve chips compensate for the uncertainty of ± 6 chips, or a distance of about ± 1758 meters. If the location of the interrogator 12 is known from its own GPS receiver and the transponder 14 must be within about 1 kilometer, which is within the communication range of the interrogator 12, then the system 10 will have no difficulty and the satellite will be within ± 1758 meters. Can be predicted. The difference in GPS signal Doppler between tow ship 22 and the boat cannot be more than 30 m / s, i.e. 0.1 ppm, and is certainly within the range of ± 0.238 ppm of the initial bandwidth. It is also possible to predict the Doppler frequency observed by the upper transponder 14.

Within the GPS tag system 10, the interrogator 12 acts as a central GPS receiver by continuing to track all visible GPS satellites and by remotely operating a set of slave correlators, ie transponders 14. Works. Interrogator 12 provides the frequency information and exact phase required by tag 14 via interrogation signal 32 and has only one expensive oscillator in the system. Since the same GPS signal 20 that prepositions the transponder 14 to the exact Doppler offset is tracked by the interrogator 12, the correlator of the transponder 14 integrates synchronously without a GPS carrier tracking loop. The transponder 14 integrates and sums the correlator output over the integration interval at each correlator tap (tap) and returns the calculated sum. The interrogator 12 uses the sampled correlation function to find the best one for the multipath-distorted correlation peak. Next, interrogator 12 determines the exact phase of the code of the transponder 14, thereby to return the pseudorange to the satellite.

According to a preferred embodiment of the GPS tag system 10, a 2-bit sampler is used to sample the data before the correlator. It is sufficient to use a 2-bit sampler. Most GPS receivers use a 1-bit sampler to avoid the need for automatic gain control (AGC) circuitry. A 1-bit sampler is prone to interference , particularly continuous wave (CW) interference . All zero crossing of the CW interference by inverting the bits, therefore, the A / D converter "and steals (stealing)" will. 2-bit sampler with AGC control of the magnitude bit density (magnitude bit density) will give sufficient margin against CW interference.

GPS tag system 10 uses a custom communications waveform (custom communication waveform), to support the design of the transponder 14. The waveform is a BPSK modulated carrier. In one embodiment, the communication link consists of 19,200 bps on a 40.68 MHz carrier that is an industrial, scientific, medical (ISM) band. Choosing a VHF carrier frequency is not essential and the system can preferably operate with UHF over the 915 MHz band, but choosing a carrier frequency of 40.68 MHz is convenient for passive standby circuit design. 100 mw is sufficient to transmit from the transponder 14 to the 1 km range at the VHF frequency. In one embodiment, the interrogator 12 operates at 200 mw to reduce the sensitivity required at the transponder 14. Another use of the GPS tag system 10 is to allow a skier to carry a transponder 14 so that if they get lost, their location can be determined. In addition, by mounting the transponder 14 on an airplane or a vehicle traveling around an airport, the controller can obtain better information to prevent a collision on the ground. Furthermore, the transponder 14 can be mounted on a golf cart, and the progress of play on the golf course can be improved.